Hands-on learning

August 31, 2011
Written by Dr. Theo Blom, University of Guelph
This is the ninth edition summary of abstracts prepared by the 2011 class taking the “Greenhouse Crop Production” course at the University of Guelph. Most students are in their third or fourth year and working towards a bachelor of science degree in agriculture (B.Sc. Agr.), and some students are working towards their BA degree.

The topics the students worked on mostly came from suggestions by industry contacts or from experiences at home. There were eight projects in total. These studies present the results of a single experiment, which had to be completed within a 10-week period (~ semester). The experiment allows the students to gain practical experience in growing plants, and also helps them determine whether research is up their alley. So the reader has to be careful about making final conclusions.

The Effect of Nitrogen on the Yield, Quality and the Toxicity of Spinach
By Brian Collins

Figure 1. The differences in growth of spinach plants irrigated with various nitrogen concentrations (ppm): 0, 25, 50, 100, 200 and 300.

■ The objective of this study was to determine the effect of various levels of nitrogen on the yield, quality and especially the nitrate levels of spinach (Spinacia oleracea L.). The latter is important to prevent methaemoglobinaemia (blue baby syndrome) and carcinogenic N-nitroso compounds when nitrate levels are high.

Spinach was grown in plastic pots filled with a non-charged soilless substrate in a greenhouse maintained at about 20ºC.

The pots were fertilized with different levels of nitrogen (ppm) – 0, 25, 50, 100, 200 and 300 – using calcium nitrate. All solutions contained a base concentration of P and K, as well as microelements.

Plant height, fresh and dry weight, chlorophyll content and nitrates were assessed at 49 days after seeding. The dry weight, chlorophyll content and nitrate accumulation initially increased linearly with the nitrogen rate and then were found to plateau at 100 ppm nitrogen. Nitrogen applications of 25 ppm caused stem elongation (compared to the 0 ppm N rate), while higher nitrogen rates did not significantly increase the height of the plants any further (Figure 1). Nitrate levels in the leaf tissue exceeded European Union regulations (3000 ppm) for nitrogen treatments above 50 ppm N.

Effects of Light Transmission of the Pot Wall on the Growth of Seed Geranium
By Katharine Phillips-Wile

Figure 2. The setup of the different pots with the five different light transmissivities.
■ There was a recent report in a European greenhouse magazine that claimed geraniums grown in transparent pots produced more cuttings than when grown in black pots.

In this experiment, I repeated this experiment by using 10 cm pots with different light transmissivities of the pot wall – 0 (black), 28, 46, 54 and 78 per cent (clear) – using seed geraniums.

All pots were the same size (10 cm) and were given the same amount of fertilizer solution. Plants were randomly spaced on a greenhouse bench and grown for about eight weeks (Figure 2). Visual observation, as well as final fresh and dry weights of top and roots, showed no differences between the five different pot types.

Interestingly, the clear pots used more water than the black pots.

The Effect of Irrigation Frequency on the Height, Aborted Buds and Leaf Area of Easter Lilies (cv. ‘Nellie White’)
By Bridget Visser and Gwen Williamson

Figure 3. Easter lilies at the time of the final height measurements (from left to right) – control (watered as needed), once, twice, or four times per week.

■ Watering frequency and irrigation volumes remain questions for lily growers. The experimental objective was to determine whether altering the watering frequency affects the quality of Easter lilies (cv. ‘Nellie White’), including factors such as height, number of (healthy and aborted) buds, leaf number, and size of leaves.

Treatments were irrigated once, twice or four times per week so that the total volume of irrigation was the same (initially 200 ml / week until visible bud stage and then increased to 400 ml / week until flowering. Control plants were watered when the substrate was considered dry.

Plant heights and bud development were measured weekly. At flowering, the final height was taken, as well as the number of buds, the number of leaves, and the total leaf area.

We found no significant difference between the treatments and control in regards to the height, (Figure 3) number of leaves or size of leaves. The three controlled irrigation treatments had one extra aborted bud compared to the control plants, but this is likely due to the location of the control plants (outside of the bench) and the high greenhouse temperature (20/20ºC) started at the beginning of February.

Water and Air Temperature on the Effectiveness of Bonzi Sprays on Petunias
By Lauren S. Ross and Emily Moeller

Figure 4. Petunias treated in a cooler at 5ºC with a 4 ppm paclobutrazol solution of 20ºC compared to control plants.

■ The purpose of this trial was to determine the impact of solution and plant temperatures on the effectiveness of the growth retardant Bonzi when applied to Picobella petunias.

Seeding occurred in plug trays and the plants were allowed to grow for approximately seven weeks before being transplanted into cell packs. Two weeks after transplantation, the trial group was split in half and one group was moved to a dark 5ºC cooler and the other remained in a dark 23.5ºC greenhouse for 24 hours prior to a single application of Bonzi (4 ppm paclobutrazol).

On the day of application, the Bonzi solution was applied at 5ºC, 10ºC or 20ºC to one cellpack each in both the greenhouse and the cooler. The control plants received de-ionized water at room temperature. The solution was allowed to dry on the plants in their respective environments for approximately five hours before all plants were returned to the original greenhouse.

The results after three weeks showed that all Bonzi treated plants were shorter than the control plants, but that there were no significant effects between the three temperatures in either the cooler or the greenhouse environment (Figure 4). All Bonzi treated plants had darker coloured leaves.

The Effect of Far-Red Lighting in Promoting Flowering of Chrysanthemums Under a Long-Day Regime
By Kevin Neil and John Patterson

Chrysanthemum morifolium is a short-day plant and thus needs long nights to initiate flowering for greenhouse production. Our experiment was started in order to determine whether far-red lighting could create artificial short-days after it was given a night interruption.

Four photoperiods were compared in a greenhouse with black-out benches using Chrysanthemum morifolium ‘Brighton’:
  • the Far-Red (FR) treatment exposed plants to eight hours of daylight, followed by four hours of darkness, then a night break of four hours using incandescent lamps and then the rest of the night (eight hours) using far-red light emitting diodes (LEDs).
  • The Night Interruption (NI) treatment was the same as FR, but without exposing the plants to far-red LEDs.
  • The Short-Day (SD) treatment provided the plants eight hours of daylight and then 16 hours of darkness.
  • The Short-Day / Far-Red (SD FR) treatment was similar to NI, but the night break was given four hours of far-red light.
All treatments received the same amount of daylight (eight hours).

Figure 5. Chrysanthemum morifolium ‘Brighton’ plants grown for eight weeks in the following lighting treatments (from left to right): FR 0.5 m, FR 4.0 m, NI, SD FR and SD.

In the FR bench, the far-red LEDs were installed at one end of the bench to create different intensities on the bench. Only the plants exposed to SD FR and SD treatments flowered, with the SD FR plants flowering about one week later than those under SD (Figure 5).

Stem length was also significantly increased on plants exposed to the far-red LED night interruption compared to the SD, but it did not prevent flowering. The FR plants did not flower at all and were generally taller than the NI plants and the plants closest to the far-red source (0.5 m) were taller than those further away (4.0 m).

This study showed that far-red light cannot replace darkness or overcome an otherwise long-day regime.

FR 0.5 m is a treatment where plants were exposed to eight hours of natural light followed by four hours of darkness, then four hours of incandescent light and then eight hours of far-red light with the plant 0.5 m away from the far-red light-emitting diodes. FR 4.0 m received the same treatment as FR 0.5 m, but had the plants 4 m from the LED lights.

NI is the lighting treatment where plants were given four hours of night interruption using incandescent lamps. SD FR had plants exposed to eight hours of natural light but given night interruption for four hours using far-red LED light. SD had plants exposed to only eight hours of light and 16 hours of darkness.

The Effectiveness of Bonzi on Height Control of Potted Tulips
By David Kralt

■ The effectiveness of paclobutrazol was tested on the scape elongation of potted tulip bulbs (‘Mary Belle’) during the forcing stage in a greenhouse maintained at 20/20ºC (D/N).

Four days after removal from the cooler, the pots received a 125 ml soil drench. Five different solutions were prepared in the following concentrations of paclobutrazol (ppm): 0, 4, 8, 16 and 32. De-ionized water was used for all solutions.

The results showed that scape length at flowering was shorter with an increased concentration of paclobutrazol.

The Effect of Blue and Red Light on Germination and Early Growth of Vegetable seedlings
By Daniel Kerfont

Figure 6. The effect of red and blue light on the height of seedlings.

■ Light is a very important environmental factor that affects plant growth and development in various important ways. Different fluencies of light regulate growth and photomorphogenic processes that are particular to a specific wavelength of light.

This study looked at the germination and early growth of various vegetable seedlings under blue (~450 nm) and red (~670 nm) light in growth chambers. The red light was provided with red LEDs and the blue light was provided by fluorescent lamps. The light intensity at the plant level was the same at 100 µmol/m2/s.

The vegetables included lettuce, tomato, beet and radish.

The results showed that the germination rate was not affected by the light quality. The major difference was the elongated stems under the red light and compact growth under the blue light (Figure 6).

Ammonium Affects Squash and Cantaloupe Stem Diameter

By Kevin Howe

■ Grafting cantaloupe onto squash rootstock has become a promising technique to increase productivity and reduce disease incidence of the cantaloupe.

Stem diameter compatibility between squash and cantaloupe for the grafting is challenged by the diameter differences between each species, squash being naturally thicker than the cantaloupe. Compatible diameters increase the success rate of grafting.

Ammonium form of nitrogen may influence stem elongation and possibly reduce stem diameter. Both squash and cantaloupe were fertilized with 0, 2, 4, 8 or 16 mmol of ammonium following germination. The results showed that both squash and cantaloupe had a reduced diameter of the stem with increased ammonium concentration. However, the squash still had a 1.5 cm larger diameter than the cantaloupe at the most optimal comparison. ■

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